This study aimed to investigate the influence of acute glycemic load on vascular endothelial function in patients with hypertension and to evaluate the protective effect of vitamins C and E during the acute glycemic phase. We randomly selected 39 hypertensive patients and 21 normal subjects and divided them into 3 groups: 75 g oral glucose (glycemic load group), 75 g glucose+0.9 g vitamin C (VC group), 75 g glucose+2 g vitamin C+0.8 g vitamin E (VC+VE group). Extravascular color Doppler ultrasound was used to detect brachial artery flow-mediated vasodilation at 0, 1, 2, and 3 h, and, at the same time, serum anti-oxidant products were measured. Basic endothelial functions in patients with hypertension were decreased in the glycemic load group (9.48±3.33 versus 13.09±6.78%, P<0.05), and was even more depressed in the hypertensive group (9.48±3.33 versus 14.20±6.48%, P<0.05). Antioxidant vitamins played a dose-dependent protective role on acute damage of endothelial function due to glycemic load. Acute high blood sugar damaged vascular endothelial functions, especially in hypertensive patients, but this effect can be reversed by large doses of vitamin C and E.